A new technique for constructing a porcelain to metal crown having a fracture resistance comparable to or greater than the fracture resistance to impact forces of the veneer cast metal crown and which overcomes many of the shortcomings of the conventional porcelain jacket crown is disclosed in U.S. Pat. Nos. 4,273,580 and 4,459,112, respectively. In accordance with U.S. Pat. No. 4,273,580, a precious metal foil, preferably a laminate of several precious metal layers, is swaged about a prepared die of a tooth to form a metal matrix upon which a veneering material such as porcelain is fired. However, unlike the conventional porcelain jacket crown, the metal matrix is not removed or separated from the veneering material but is instead retained as a metal coping for the finished porcelain jacket crown. The metal coping is employed as an understructure for the jacket crown comparable in function to the cast metal understructure in the conventional porcelain to metal cast crown.
The physical strength of the metal coping may be substantially enhanced and the ease of preparing the restoration greatly simplified by converting the metal foil starting material into a preformed coping of predetermined geometry as taught and described in U.S. Pat. No. 4,459,112 referred to above. The metal foil starting material is cut into a circular segment and folded to form multiple pleats which are uniformly spaced apart and preferably extend radially from a central unfolded area. This multiple fold geometry makes it easy to adapt the preformed coping to the die without the need for superior skill and craftsmanship and even more importantly increases the rigidity and strength of the coping.
In each of the patents referred to above, the preformed coping is formed from a metal foil starting material preferably in the form of a laminated composite of at least two layers of precious metal or precious metal alloy. One of the layers is composed of a high fusing temperature precious metal such as platinum whereas the other is composed of a lower fusing temperature gold based metal composition substantially or entirely of gold.
The ability to adapt and swage the preformed coping to the die depends upon the flexibility and workability of the composite of metal layers from which the coping is formed and, in turn, is a measure of the softness of the metal foil. Conversely, the strength of the coping after it is swaged and removed from the die is dependent upon the hardness and rigidity of the coping. The hardness or softness of a metal is determined by measuring its resistance to permanent identation. A hardness number is assigned to the material using any one of several conventional hardness tests such as the Vickers hardness test which uses a diamond pyramid indenter. A dental coping should accordingly be sufficiently soft to adapt to the die and provide a close marginal fit and yet be hard and rigid after adaptation to provide strength.